Decoder providing separate clock and enable for scan path segments

ABSTRACT

Scan architectures are commonly used to test digital circuitry in integrated circuits. The present invention describes a method of adapting conventional scan architectures into a low power scan architecture. The low power scan architecture maintains the test time of conventional scan architectures, while requiring significantly less operational power than conventional scan architectures. The low power scan architecture is advantageous to IC/die manufacturers since it allows a larger number of circuits (such as DSP or CPU core circuits) embedded in an IC/die to be tested in parallel without consuming too much power within the IC/die. Since the low power scan architecture reduces test power consumption, it is possible to simultaneously test more die on a wafer than previously possible using conventional scan architectures. This allows wafer test times to be reduced which reduces the manufacturing cost of each die on the wafer.

This application is a divisional of application Ser. No. 13/657,082,filed Oct. 22, 2012, now U.S. Pat. No. 8,539,294, issued Sep. 17, 2013;

Which was a divisional of application Ser. No. 13/091,760, filed Apr.21, 2011, now U.S. Pat. No. 8,321,729, issued Nov. 27, 2012;

Which was a divisional of application Ser. No. 12/962,283, filed Dec. 7,2010, now U.S. Pat. No. 7,954,030, issued May 31, 2011;

Which was a divisional of application Ser. No. 12/512,207, filed Jul.30, 2009, now U.S. Pat. No. 7,870,451, issued Jan. 11, 2011;

Which was a divisional of application Ser. No. 11/684,042, filed Mar. 9,2007, now U.S. Pat. No. 7,617,429, issued Nov. 10, 2009;

Which was a divisional of application Ser. No. 10/886,189, filed Jul. 6,2004, now U.S. Pat. No. 7,219,284, issued May 15, 2007;

Which was a divisional of application Ser. No. 09/997,462, filed Nov.29, 2001, now U.S. Pat. No. 6,766,487, issued Jul. 20, 2004;

which claimed priority from Provisional Application No. 60/250,646,filed Dec. 1, 2000.

This disclosure relates to and incorporates by reference patentapplication Ser. No. 60/187,972, filed Mar. 9, 2000, now U.S. Pat. No.6,769,080.

PRIOR ART DESCRIPTION

FIG. 1 illustrates a conventional scan architecture that a circuit 100can be configured into during test. In the normal functionalconfiguration, circuit 100 may be a functional circuit within IC, but intest configuration it appears as shown in FIG. 1. Scan architectures canbe applied at various circuit levels. For example, the scan architectureof FIG. 1 may represent the testing of a complete IC, or it mayrepresent the testing of an embedded intellectual property coresub-circuit within an IC, such as a DSP or CPU core sub-circuit. Thescan architecture includes a scan path circuit 104, logic circuitry tobe tested 108, and connection paths 112-120 to a tester 110. Tester 110operates to; (1) output control to operate scan path 104 via controlpath 114, (2) output serial test stimulus patterns to scan path 104 viascan input path 118, (3) input serial test response patterns from scanpath 104 via scan output path 120, (4) output parallel test stimuluspatterns to logic 108 via primary input path 112, and (5) input paralleltest response patterns from logic 108 via primary output path 116. Scanpath 104 operates, in addition to its scan input and scan output modesto tester 110, to output parallel test stimulus patterns to logic 108via path 122, and input parallel response patterns from logic 108 viapath 124.

Typically tester 110 is interfaced to the scan architecture by probingthe die pads at wafer level, or by contacting package pins after the dieis assembled into a package. While tester 110 connections to the primaryinputs 112 and primary outputs 116 of logic 108 are shown, the primaryinput and output connections could be achieved by augmentation of scanpath 104. For example, scan path 104 could be lengthened to includeboundary scan cells located on each primary input and primary output oflogic 108. The boundary scan cells would provide primary inputs to andprimary outputs from logic 108, via widened stimulus and response busses122 and 124, respectively. In some instances, logic 108 may besufficiently tested by scan path 104 such that it is not necessary toprovide primary inputs to and outputs from logic 108 via the tester orvia the above described augmentation of scan path 104. For example, ifthe amount of logic 108 circuitry made testable by the use of scan path104 in combination with the primary inputs and outputs is very smallcompared to the amount of logic 108 circuitry made testable by the scanpath 104 alone, then the primary input and output connections to logic108 may removed without significantly effecting the test of logiccircuitry 108. To simplify the description of the prior art andfollowing description of the present invention, it will be assumed thatlogic circuit 108 is sufficiently tested using only scan path 104, i.e.the primary inputs 112 and primary outputs 116 are not required.However, it is clear that primary input and output connections to thetester or to an augmented scan path 104, as described above, could beused as well.

FIG. 2 illustrates an example of a conventional scan cell that could beused in scan path 104. (Note: The optional scan cell multiplexer 218 andconnection paths 220 and 224, shown in dotted line, will not bediscussed at this time, but will be discussed later in regard to FIGS. 7and 8.) The scan cell consists of a D-FF 204 and a multiplexer 202.During normal configuration of the circuit 100, multiplexer 202 and D-FF204 receive control inputs SCANENA 210 and SCANCK 212 to input andoutput functional data to logic 108 via paths 206 and 216, respectively.In the normal configuration, the SCANCK to D-FF 204 is typically afunctional clock, and the SCANENA signal is set such that the D-FFalways clocks in functional data from logic 108 via path 206. During thetest configuration of FIG. 2, multiplexer 202 and D-FF 204 receivecontrol inputs SCANENA 210 and SCANCK 212 to capture test response datafrom logic 108 via path 206, shift data from scan input path 208 to scanoutput path 214, and apply test stimulus data to logic 108 via path 216.In the test configuration, the SCANCK to D-FF 204 is the test clock andthe SCANENA signal is operated to allow capturing of response data fromlogic 108 and shifting of data from scan input 208 to scan output 214.During test configuration, SCANENA is controlled by tester 110 via path114. SCANCK may also be controlled by the tester, or it may becontrolled by another source, for example a functional clock source. Forthe purpose of simplifying the operational description, it will beassumed that the SCANCK is controlled by the tester.

The scan inputs 208 and scan outputs 214 of multiple scan cells areconnected to form the serial scan path 104. The stimulus path 216 andresponse path 206 of multiple scan cells in scan path 104 form thestimulus bussing path 122 and response bussing path 124, respectively,between scan path 104 and logic 108. From this scan cell description, itis seen that the D-FF is shared between being used in the normalfunctional configuration and the test configuration. During scanoperations through scan path 104, the stimulus outputs 216 from eachscan cell ripple, since the stimulus 216 path is connected to the scanoutput path 214. This ripple causes all the inputs to logic 108 toactively change state during scan operations. Rippling the inputs tologic 108 causes power to be consumed by the interconnect and gatingcapacitance in logic 108.

FIG. 3 illustrates a simplified example of how tester 110 operates 300the scan architecture during test. Initially the tester will outputcontrol on path 114 to place the scan architecture in an idle state 302.Next, the tester outputs control on path 114 to place the scanarchitecture in an operate scan path state 304. In the operate scan pathstate, the tester outputs control to cause the scan path to acceptstimulus data from the tester via path 118 and to output response datato the tester via path 120. The tester maintains the operate scan pathstate until the scan path has been filled with stimulus data and emptiedof response data. From the operate scan path state, the tester outputscontrol on path 114 to place the scan architecture in a capture responsedata state 306. In the capture response data state, the tester outputscontrol to cause the scan path to load response data from logic 108 viapath 124. From the capture response data state, the tester outputscontrol on path 114 to cause the scan architecture to re-enter theoperate scan path state. The process of entering the operate scan pathstate 304 to load stimulus into the scan path and empty response fromthe scan path, then passing through the capture response state 306 toload new response data from logic 108 repeats until the end of test. Atthe end of test the tester outputs control to cause the scanarchitecture to re-enter the idle state 302.

FIG. 4 illustrates a timing example of how tester 110 outputs SCANENAand SCANCK signals to scan path 104 during scan operations. In thisexample, a high to low transition on SCANENA, at time 406, incombination with SCANCKs occurring during time interval 402, causesstimulus data from the tester to be input to the scan path via path 118while response data is output from the scan path to the tester via path120. A low to high transition on SCANENA, at time 408, in combinationwith a SCANCK at time 404, causes response data from logic 108 to beloaded into the scan path. Time interval 402 relates to operate scanpath state 304 and time interval 404 relates to capture response state306 of FIG. 3. As seen in the timing and operation diagrams of FIGS. 3and 4, the time interval sequences 404 (i.e. state 306) and 402 (i.e.state 304) cycle a sufficient number of times during test to input allstimulus to and obtain all response from logic 108.

From the scan architecture described in regard to FIGS. 1, 2, 3, and 4it is seen that the stimulus 122 outputs ripple the inputs to logic 108as data shifts through the scan path 104 during scan operations.Rippling the inputs of logic 108 causes simultaneous charging anddischarging of capacitance's associated with the interconnects and gatesof logic 108. For example, each scan cell stimulus output 216 to logic108 charges and discharges a certain amount of capacitance within logic108 at a frequency related to the data bits being scanned through thescan cell. While each scan cell stimulus output may only be directlyinput to a few gates within logic 108, each of the gates have outputsthat fanout to inputs of other gates, and the outputs of the other gatesagain fanout to inputs of still further gates, and so on. Thus atransition on the stimulus output of a single scan cell may initiatehundreds of transitions within logic 108 as a result of the abovementioned signal transition fanout. Each of the transitions charge ordischarge a portion of the total capacitance with logic 108 andtherefore contribute to power consumption within logic 108.

The individual power (Pi) consumed by the rippling of a given scan celloutput 216 can be approximated by CV.sup.2F, where C is the capacitancebeing charged or discharged by the scan cell output (i.e. thecapacitance of the above mentioned signal transition fanout), V is theswitching voltage level, and F is the switching frequency of the scancell output. The total power (Pt) consumed by simultaneously scanningall the scan cells in scan path 104 is approximately the sum of theindividual scan cell powers, i.e. Pt=Pi.sub. 1+Pi.sub.2+ . . . Pi.sub.N.The total power consumed by circuit 100 when it is configured into thescan architecture of FIG. 1 can exceed the power consumed by circuit 100when it is configured into its normal functional mode. This can beunderstood from the fact that, during normal functional mode of circuit100, not all the D-FFs 204 simultaneously operate, as they do duringscan operations occurring during the above described scan testoperation. Further if an IC contained multiple circuits 100, the test ofthe IC may require testing each circuit 100 individually due to theabove described test power consumption restriction. This lengthens thetest time of the IC, which increases the cost to manufacture the IC.

A first known method of reducing power consumption during test operationis to insert blocking circuitry, such as a gate, into the stimulus paths216 of each scan cell, such that during scan operations the inputs tologic 108 are blocked from the effect of the scan ripple. The problemwith the first method is that it adds an undesirable delay (i.e. theblocking circuit delay) in the stimulus paths 216 between D-FFs 204 andlogic 108. This delay can negatively effect the performance of circuit100 when it is configured into its normal functional mode. A secondknown method is to reduce the scan clock rate, such that the ripplefrequency (F) is reduced. The problem with the second method is that itincreases the test time since scan operations are performed at thereduced scan clock rate.

Today, there are a number of test synthesis vendor tools that cansynthesize and insert scan architectures into ICs, similar in structureto the scan architecture shown in FIG. 1. The use of such “push-button”scan insertion tools is an attractive alternative to customized scandesigns since it is an automated process. As will be described, thepresent invention provides a method of adapting these synthesized scanarchitectures such that they may operate in a desired low power mode.The process of adapting scan architectures for low power operation isalso easily automated.

BRIEF DESCRIPTION OF THE VIEWS OF THE DRAWING FIGURES

FIG. 1 depicts a conventional scan architecture.

FIG. 2 depicts a conventional scan cell.

FIG. 3 depicts a simplified example of a tester operating a scanarchitecture.

FIG. 4 depicts a timing diagram of SCANENA and SCANCK.

FIG. 5 depicts a conventional scan architecture adopted to the low powerscan architecture of this disclosure.

FIG. 6 depicts a state diagram.

FIG. 7 depicts an example adaptor circuit.

FIG. 8 depicts a timing diagram.

FIG. 9 depicts a schematic diagram of a scan path receiving stimulusframes from a tester.

FIG. 10 depicts a circuit configured for testing using a conventionalparallel scan architecture.

FIG. 11 depicts a state diagram.

FIG. 12 depicts the circuit of FIG. 10 adapted for low power operation.

FIG. 13 depicts a state diagram.

FIG. 14 depicts a circuit configured into a conventional scan controllerbased scan architecture.

FIG. 15 depicts a state diagram.

FIG. 16 depicts modifications to the circuit of FIG. 14 to achieve lowpower operation.

FIG. 17 depicts a state diagram.

FIG. 18 depicts an IC having three IP core circuits configures forsimultaneous parallel scan testing.

FIG. 19 depicts an IC having three IP core circuits configures forsimultaneous parallel scan testing.

FIG. 20 depicts the IC of FIG. 18 after the scan paths have beenconfigured into low power scan paths.

FIG. 21 depicts the IC of FIG. 19 after the scan paths have beenconfigured into low power scan paths.

FIG. 22 depicts an alternative way of controlling low power scansegments.

FIG. 23 depicts a state diagram.

FIG. 24 depicts a decode logic circuit.

FIG. 25 depicts a timing diagram.

FIG. 26 depicts a single decode logic with low power scan paths.

FIG. 27 depicts a state diagram.

INVENTION DESCRIPTION

The present invention described below provides a method of adaptingsynthesized scan architectures to achieve a low power mode of operation.The process of adapting scan architectures for low power operation isachieved without the aforementioned problems of; (1) having to insertblocking circuitry in the stimulus paths which adds signal delays, and(2) having to decrease the scan clock rate which increases test time.Furthermore, as will be described in more detail later, the process ofadapting scan architectures for low power operation is achieved withouthaving to modify the stimulus and response test patterns which areautomatically produced by scan architecture synthesis tools.

The circuits and processes disclosed in this patent are used inmanufacturing to test and ensure proper operation of the integratedcircuit products before sale. The circuits and processes disclosed inthis patent can also be used after the sale of the integrated circuitproducts to test and ensure the continued proper operation of theintegrated circuit products and possibly to develop and test softwareproducts associated with the integrated circuit products.

FIG. 5 illustrates the scan architecture of FIG. 1 after it has beenadapted into the low power scan architecture of the present invention.The changes between the FIG. 1 scan architecture and the FIG. 5 lowpower scan architecture involve modification of scan path 104 into scanpath 502, and the insertion of an adaptor circuit 504 in the controlpath 114 between tester 110 and scan path 502.

Adapting scan path 104 into scan path 502 involves reorganizing scanpath 104 from being a single scan path containing all the scan cells(M), into a scan path having a desired number of selectable separatescan paths. In FIG. 5, scan path 502 is shown after having beenreorganized into three separate scan paths A, B, and C 506-510. It isassumed at this point in the description that the number of scan cells(M) in scan path 104 is divisible by three such that each of the threeseparate scan paths A, B, and C contains an equal number of scan cells(M/3). The case where scan path 104 contains a number of scan cells (M)which, when divided by the number of desired separate scan paths, doesnot produce an equal number of scan cells in each separate scan pathwill be discussed later in regard to FIG. 9.

Scan paths A, B, and C are configured as follows inside scan path 502.The serial input of each scan path A, B, and C is commonly connected totester 110 via connection path 118. The serial output of scan path A isconnected to the input of a 3-state buffer 512, the serial output ofscan path B is connected to the input of a 3-state buffer 514, and theserial output of scan path C is connected to the input of a 3-statebuffer 516. The outputs of the 3-state buffers 512-516 are commonlyconnected to tester 110 via connection path 120. Scan paths A, B, and Ceach output an equal number of parallel stimulus inputs 526, 530, 534 tologic 108, and each input an equal number of parallel response outputs524, 528, 532 from logic 108. The number of stimulus output signals tologic 108 in FIGS. 1 and 5 is the same. The number of response inputsignals from logic 108 in FIGS. 1 and 5 is the same. Scan path A andbuffer 512 receive control input from adaptor 504 via bus 518, scan pathB and buffer 514 receive control input from adaptor 504 via bus 520, andscan path C and buffer 516 receive control input from adaptor 504 viabus 522.

Adaptor 504 is connected to scan paths A, B, C via busses 518-522 and totester 110 via bus 114. The purpose of the adaptor is to intercept thescan control output 114 from tester 110 and translate it into a sequenceof separate scan control outputs 518-522 to scan paths A, B, and C,respectively. Each of the separate scan control outputs 518-522 are usedto operate one of the scan paths A, B, and C.

FIG. 6 illustrates a simplified example of the combined operation 600 ofthe tester 110 and adaptor 504 during test. The operation of tester 110is the same as previously described in regard to FIG. 3. When the testertransitions to the operate scan path state 304, it begins outputtingcontrol to adaptor 504 via path 114. The adaptor responds to the testercontrol input by translating it into a sequence of separate controloutputs 518, 520, and 522 to scan paths A, B, and C. As indicated inadaptor operation block 602, the adaptor first responds to control 114during adaptor operate state 604 to output control 518, which enablesbuffer 512 and operates scan path A to input stimulus data from tester110 via path 118 and output response data to tester 110 via path 120.After scan path A is filled with stimulus and emptied of response,adaptor 504 responds to control 114 during operation state 606 to outputcontrol 520, which enables buffer 514 and operates scan path B to inputstimulus data from tester 110 via path 118 and output response data totester 110 via path 120. After scan path B is filled with stimulus andemptied of response, adaptor 504 responds to control 114 duringoperation state 608 to output control 522, which enables buffer 516 andoperates scan path C to input stimulus data from tester 110 via path 118and output response data to tester 110 via path 120. After scan paths A,B, and C have been filled and emptied, the tester 110 transitions fromthe operate state 304, through the capture state 306, and back to theoperate state 304. During this transition, the adaptor is idle duringthe capture state 306, but resumes its scan control sequencing operationwhen the operate state 304 is re-entered. This process of sequentiallyscanning scan paths A, B, and C, then performing a capture operation toload response data repeats until the test has been performed and tester110 enters the idle state 302.

During the sequencing of the operation states 604-608, only one of thebuffers 512-516 are enabled at a time to output response data to tester110. Also, the sequencing of the adaptor operation states 604-608 occursin a seamless manner such that the stimulus data from the tester 110 isinput to scan path 502 as it was input to scan path 104, and theresponse data to tester 110 is output from scan path 502 as it wasoutput from scan path 104. To the tester, the behavior of the scan path502 and adaptor 504 combination is indistinguishable from the behaviorof the scan path 104 in FIG. 1. Thus the test time of the logic 108 inFIG. 5 is the same as the test time of logic 108 in FIG. 1.

From the above description, it is seen that only a subset (i.e. subset A526, B 530, or C 534) of the stimulus input bus 122 to logic 108 isallowed to ripple at any given time during the adaptor operated scanoperation of FIGS. 5 and 6. In contrast, the entire stimulus input bus122 to logic 108 ripples during the tester operated scan operation ofFIGS. 1 and 3. Since, using the present invention, only a subset of thestimulus inputs to logic 108 are allowed to ripple at any one time, lessof the aforementioned interconnect and gating capacitance of logic 108is simultaneously charged and discharged during scan operations. Byreducing the amount of logic 108 capacitance being simultaneouslycharged and discharged during scan operations, the power consumed bylogic 108 is advantageously reduced by the present invention.

Example Adaptor Circuit

FIG. 7 illustrates an example adaptor circuit 504 implementation.Adaptor 504 inputs the SCANCK 212 and SCANENA 210 signals from tester110, via bus 114. Adaptor 504 outputs SCANCK-A signal 712, SCANCK-Bsignal 714, SCANCK-C signal 716, ENABUF-A signal 718, ENABUF-B signal720, ENABUF-C signal 722, and the SCANENA signal 210. The SCANENA signal210 is connected to all scan cell 200 multiplexers 202 as shown in FIG.2. The SCANCK-A signal 712 is connected, in substitution of SCANCKsignal 212, to all scan cell 200 D-FF 204 clock inputs of scan path A.The SCANCK-B signal 714 is connected, in substitution of SCANCK signal212, to all scan cell 200 D-FF 204 clock inputs of scan path B. TheSCANCK-C signal 716 is connected, in substitution of SCANCK signal 212,to all scan cell 200 D-FF 204 clock inputs of scan path C. The ENABUF-Asignal 718 is connected to the enable input of buffer 512. The ENABUF-Bsignal 720 is connected to the enable input of buffer 514. The ENABUF-Csignal 722 is connected to the enable input of buffer 516.

Adaptor 504 includes a state machine 702, counter 704, and gates706-710. During functional mode of circuit 500, SCANENA is high asindicated at time 810 in the adaptor timing diagram of FIG. 8. WhileSCANENA is high, state machine 702 outputs control signals 724-728 thatenable SCANCK to pass through gates 706-710 to functionally clock allD-FFs 204 of scan paths A, B, and C, via SCANCK-A, SCANCK-B, andSCANCK-C. In this example, the SCANCK is assumed to be the functionalclock during the functional mode of circuit 500, and the test clockduring test mode of circuit 500. While SCANENA is high, state machine702 outputs control signals 718-722 to disable buffers 512-516. The scanoperation mode is entered by SCANENA going low as indicated at time 812in FIG. 8. SCANENA goes low when tester 110 transitions from the idlestate 302 to the operate state 304 as seen in FIG. 6.

At the beginning of the scan operation mode, the state machineinitializes counter 704 via control (CTL) signals 730 and disables scanaccess to scan paths B and C by disabling SCANCK gates 708 and 710 viasignals 726 and 728, and enables scan access to scan path A by; (1)enabling SCANCK gate 706 via signal 724, and (2) enabling buffer 512 viasignal 718. Scan access of scan path A occurs over time interval 802 ofFIG. 8. During time interval 802, scan path A is accessed to loadstimulus data from tester 110 via path 118 and unload response to tester110 via path 120. While scan path A is being accessed, the state machineoperates counter 704 via control signals 730 to determine the number(M/3) of SCANCK-A's to output to scan path A. When the counter reaches acount, indicative of scan path A receiving the correct number (M/3)SCANCK-A inputs, it outputs a first count complete 1 (CC1) signal 732 tostate machine 702.

In response to the first CC1 signal, the state machine initializescounter 704 via control signals 730 and disables scan access to scanpath A and C, and enables scan access to scan path B over time interval804. The state machine enables scan access to scan path B by; (1)enabling SCANCK gate 708 via signal 726, and (2) enabling buffer 514 viasignal 720. While scan path B is being accessed, the state machineoperates counter 704 via control signals 730 to determine the number ofSCANCK-B's to output to scan path B. When the counter reaches a count,indicative of scan path B receiving the correct number (M/3) SCANCK-Binputs, it outputs a second count complete 1 (CC1) signal 732 to statemachine 702.

In response to the second CC1 signal, the state machine initializescounter 704 via control signals 730 and disables scan access to scanpath A and B, and enables scan access to scan path C over time interval806. The state machine enables scan access to scan path C by; (1)enabling SCANCK gate 710 via signal 728, and (2) enabling buffer 516 viasignal 722. While scan path C is being accessed, the state machineoperates counter 704 via control signals 730 to determine the number ofSCANCK-C's to output to scan path C. When the counter reaches a count,indicative of scan path C receiving the correct number (M/3) SCANCK-Cinputs, it outputs a third count complete 1 (CC1) signal 732 to statemachine 702.

In response to the third CC1 signal, the state machine disables allbuffers 512-516 via signals 718-722 and enables gates 706-710 to passthe SCANCK to all scan cells of scan paths A, B, and C. Since scan pathsA, B, and C were assumed to contain equal numbers of scan cells (M/3)with the sum of the scan cells in scan paths A, B, and C being equal tothe number of scan cells (M) in scan path 104, the third CC1 signaloccurs one SCANCK prior to tester 110 setting the SCANENA signal high,at time 814, during its transition from the operate state 304 to thecapture state 306 in FIG. 6. While SCANENA is high, at time 808, allscan paths A, B, and C receive a SCANCK, causing them to load responsedata from logic 108 of FIG. 5. Following the response data loadoperation at time 808, SCANENA, from tester 110, returns low at time 812and the above described sequence of separately accessing scan paths A,B, and C repeats until the test completes and tester 110 transitionsback to idle state 302 of FIG. 6.

Contrasting the scan timing diagrams of FIGS. 4 and 8, it is seen thattester 110 provides the same SCANENA timing for both diagrams. Forexample, (1) the SCANENA high to low transition at time 406 in FIG. 4 isthe same SCANENA high to low transition at time 812 in FIG. 8, (2) theSCANENA low to high transition at time 408 in FIG. 4 is the same SCANENAlow to high transition at time 814 in FIG. 8, (3) the same number ofSCANCKs occur between time 406/812 and time 408/814 in both diagrams,and (4) the same response load SCANCK occurs at time 404 in FIG. 4 andat time 808 in FIG. 8. The difference between the two timing diagrams isseen in the way the adaptor 504 sequentially applies a burst of M/3SCANCKs to scan paths A, B, and C during time intervals 802, 804, and806, respectively, such that only one of the scan paths is accessed at atime.

While the example adaptor circuit of FIG. 7 has been described using agated clocking scheme to control access to the scan cells 200 of scanpaths A, B, and C, other example designs of adaptor 504 may be used tocontrol access to other types of scan cells used in scan paths A, B, andC as well. For example, the scan cells 200 of FIG. 2 could be designedto include a state hold multiplexer 218 between the output ofmultiplexer 202 and input to D-FF 204. The state hold multiplexer 218could be controlled, via a connection 220 to the ENACK-A 724, ENACK-B726, and ENACK-C 728 signals from state machine 702, such that itprovides a connection 222 between the output of multiplexer 202 and theD-FF input, or it provides a state hold connection 224 between theoutput of DFF 204 and the input to D-FF 204. If this type of scan cell200 were used in scan paths A, B, and C, the SCANCK 212 could bedirectly routed to all the D-FF 204 clock inputs instead of being gatedto the D-FF 204 clock inputs via the SCANCK-A, SCANCK-B, and SCANCK-Csignals as described for adaptor 504 of FIG. 7. The adaptor 504 would bemodified to operate the state holding scan cells by eliminating thegates 706-710 and the SCANCK-A, SCANCK-B, and SCANCK-C outputs, andproviding as outputs the ENACK-A 724, ENACK-B 726, and ENACK-C 728signals from state machine 702. The ENACK-A output would be connected ascontrol input 220 to the state hold multiplexers 218 in the scan cellsof scan path A. The ENACK-B output would be connected as control input220 to the state hold multiplexers 218 in the scan cells of scan path B.The ENACK-C output would be connected as control input 220 to the statehold multiplexers 218 in the scan cells of scan path C.

During functional and response capture operations, the ENACK-A, ENACK-B,and ENACK-C outputs from the modified adaptor 504 would be set to enablea connection between the response signal 206 and input to D-FF 204 ofeach scan cell, via multiplexer 202 and the state hold multiplexer 218.During scan operations to scan path A (timing interval 802), the ENACK-Band ENACK-C outputs would be set to place the scan cells of scan paths Band C in their state hold connection configuration, and ENACK-A would beset to form a connection between the scan input 208 and input to D-FF204 of the scan cells in scan paths A, to allow scan access of scan pathA. During scan operations to scan path B (timing interval 804), theENACK-A and ENACK-C outputs would be set to place the scan cells of scanpaths A and C in their state hold connection configuration, and ENACK-Bwould be set to form a connection between the scan input 208 and inputto D-FF 204 of the scan cells in scan paths B, to allow scan access ofscan path B. During scan operations to scan path C (timing interval806), the ENACK-A and ENACK-B outputs would be set to place the scancells of scan paths A and B in their state hold connectionconfiguration, and ENACK-C would be set to form a connection between thescan input 208 and input to D-FF 204 of the scan cells in scan paths C,to allow scan access of scan path C.

The modified adaptor 504 and state hold type scan cells described aboveoperate to achieve the low power mode of scan access to scan paths A, B,and C as previously described with the original adaptor 504 and scancell 200. The difference between the two adaptor/scan cell combinationsdescribed above is that the original adaptor/scan cell combinationoperates in a gated clock mode (i.e. uses gated clocks SCANCK-A,SCANCK-B, and SCANCK-C) and the modified adaptor/scan cell combinationoperates in a synchronous clock mode C (i.e. uses the SCANCK).

Scan Path Adaptation

As mentioned previously, test synthesis tools exist that are capable ofautomatically instantiating scan architectures similar to the one shownin FIG. 1. These tools are capable of analyzing logic 108 and itsstimulus and response interface to scan path 104 to determine whatstimulus test pattern data needs to input from tester 110 to logic 108via scan path 104 and what response test patterns data is expected to beoutput to tester 110 from logic 108 via scan path 104. To reduce theeffort required to adapt the synthesized scan architecture of FIG. 1into the low power scan architecture of FIG. 5, the scan path adaptationprocess described below is preferably performed.

In FIG. 9, scan'path 104 is shown receiving stimulus frames 920 fromtester 110 via connection 118 and outputting response frames 922 totester 110 via connection 120. The term “frame” simply indicates thenumber of scan bits (M) required to fill the scan path 104 with stimulusdata from tester 110 and empty the scan path 104 of response data totester 110 during the operate state 304 of FIG. 3. The test may requirea large number of stimulus and response frame communications to testlogic 108. To achieve the low power mode of operation of the presentinvention, it is desired to reorganize scan path 104 into a plurality ofseparate scan paths. In this example, the reorganization of scan path104 results in the previously described scan path 502, which containsthree separate scan paths 506-510. It is also desired to adapt scan path104 into scan path 502 in such a way as to avoid having to make anymodifications to the stimulus and response test pattern frames 920 and922.

As previously mentioned in regard to FIG. 5, the number (M) of scancells in scan path 104, is assumed divisible by three such that scanpath 104 can be seen to comprise three separate scan segments A, B, andC, each scan segment containing a third (M/3) of the scan cells (M) inscan path 104. Scan segment A of 104 contains a subset 912 of thestimulus and response signals of the overall stimulus and responsebusses 122 and 124 respectively. Scan segment B of 104 contains a subset910 of the stimulus and response signals of the overall stimulus andresponse busses 122 and 124 respectively. Scan segment C of 104 containsa subset 912 of the stimulus and response signals of the overallstimulus and response busses 122 and 124 respectively.

Each stimulus scan frame 920 scanned into scan path 104 from tester 110can be viewed as having bit position fields [CBA] that fill scansegments C, B, and A, respectively. For example, following a scanoperation, bit position field A is loaded into segment A, bit positionfield B is loaded into segment B, and bit position field C is loadedinto segment C. Likewise, each response scan frame 922 scanned from scanpath 104 to tester 110 can be viewed as having bit position fields [CBA]that empty scan segments C, B, and A, respectively. For example,following a scan operation, bit position field A is unloaded fromsegment A, bit position field B is unloaded from segment B, and bitposition field C is unloaded from segment C. To insure that the stimulus920 and response 922 frames are reusable when scan path 104 isreorganized into the low power configuration, the reorganization processoccurs as described below.

Scan path 104 segment A is configured as a separate scan path A 506, asindicated by the dotted line 914. Scan path 104 segment B is configuredas a separate scan path B 508, as indicated by the dotted line 916. Scanpath 104 segment C is configured as a separate scan path C 510, asindicated by the dotted line 918. The scan inputs to scan paths A, B,and C 506-510 are connected to tester 110 via connection 118. The scanoutputs from scan paths A, B, and C 506-510 are connected, via thepreviously described 3-state buffers 512-516, to tester 110 viaconnection 120. Each separate scan path 506-510 maintains the samestimulus and response bussing connections 908-912 to logic 108.

Operating the reorganized scan path 502 using the tester 110 used tooperate scan path 104 results in the following behavior. This behaviorassumes adaptor 504 has been inserted between the tester 110 and scanpath 502, to control scan path 502 as described in FIGS. 5, 6, 7, and 8.During input and output of stimulus and response frames [CBA] 920 and922 respectively, (1) stimulus bit field A is directly loaded into scanpath A from tester 110 via path 118 as response bit field A is directlyunloaded from scan path A to tester 110 via path 120, (2) stimulus bitfield B is directly loaded into scan path B from tester 110 via path 118as response bit field B is directly unloaded from scan path B to tester110 via path 120, and (3) stimulus bit field C is directly loaded intoscan path C from tester 110 via path 118 as response bit field C isdirectly unloaded from scan path C to tester 110 via path 120. As seenfrom this description, when scan path 104 is reorganized into scan path502 as described, scan path 502 can use the same stimulus and responseframes originally intended for use by scan path 104. Thus nomodifications are necessary to the stimulus and response test patternframes produced by the test synthesis tool.

In the case where scan path 104 contains a number of scan cells (M) thatis not equally divisible by the desired number of separate scan paths(N) in scan path 502, the length of one of the separate scan paths canbe adjusted to compensate scan path 502 for proper input and output ofthe scan frames 920-922. For example, if the number of scan cells (M) inscan path 104 is not equally divisible by the number of separate scanpaths (N) required to achieve a desired low power mode of operation, Mcan be increased by adding a value (Y) such that M+Y is equallydivisible by N. Once this is done, N separate scan paths may be formed.N−1 of the separate scan paths will have a length (M+Y)/N and one of theseparate scan paths will have a length of ((M+Y)/N)−Y. For example, ifscan path 104 had 97 scan cells (M), scan path A and B of 502 would eachbe configured to contain 33 scan cells [(M+Y)/N=(97+2)/3=33], while scanpath C would be configured to contain 31 scan cells[((M+Y)/N)−Y=((97+2)/3)−2=31]. In this example, the scan frame 920-922[CBA] segments would be seen as; segment A=33 bits, segment B=33 bits,and segment C=31 bits.

When scan path 502 is formed to include the scan frame compensationtechnique described above, the operation of adaptor 504 is adjusted soit can properly control the compensated scan path 502. In FIGS. 7 and 8,the adaptor 504 circuit and operation was described in detail. Assumingthe adaptor timing diagram in FIG. 8 is being used to communicate scanframes to a scan path 502 consisting of the above mentioned 33-bit scanpath A, 33-bit scan path B, and 31-bit scan path C, the followingchanges are required to adaptor 504. Adaptor state machine 702 continuesto monitor the CC1 732 output from counter 704, as previously described,to determine when to stop 33-bit scan operations to scan paths A and Bat timing intervals 802 and 804, respectively, in FIG. 8. However, sincethe scan timing interval 806 to scan path C is different from the scantiming intervals 802 and 804, the state machine operation is altered towhere it monitors the count complete 2 (CC2) output 734 from counter 704to stop the 31-bit scan operation to scan path C. The CC2 734 output isdesigned to indicate when the 31-bit scan operation to scan path Cshould be stopped, whereas the CC1 732 is designed to indicate when the33-bit scan operation to scan paths A and B should be stopped.

Parallel Scan Architectures

FIG. 10 illustrates circuit 1000 which has been configured for testingusing a conventional parallel scan architecture. As with the previoussingle scan architecture of FIG. 1, parallel scan architectures may besynthesized and automatically inserted into ICs to serve as embeddedtesting mechanisms. The parallel scan architecture includes separatescan paths 1-N 1010-1016 and an interface to tester 1008. Duringfunctional mode of circuit 1000, the D-FFs 204 of scan paths 1-N areconfigured to operate with logic 1006 to provide the circuit 1000functionality. During test mode, the D-FFs 204 of scan path 1-N areconfigured to operate with tester 1008 to provide testing of logic 1006.Scan paths 1-N receive response from logic 1006 via paths 1040-1046, andoutput stimulus to logic 1006 via paths 1048-1054. Scan paths 1-Nreceive serial stimulus from tester 1008 via paths 1010-1024, and outputserial response to tester 1008 via paths 1026-1032. Scan paths 1-Nreceive control input from tester 1008 via path 1034.

When circuit 1000 is first placed in the test configuration of FIG. 10,the parallel scan architecture will be controlled, by tester 1008, to bein the idle state 1102 of the test operation diagram 1100 in FIG. 11.From the idle state 1102, tester 1008 will transition the parallel scanarchitecture into the operate scan paths 1-N state 1104. During theoperate state 1104, tester 1008 outputs control to scan paths 1-Ncausing the scan paths to input stimulus from tester 1008 via paths1018-1024 and output response to tester 1008 via paths 1026-1032. Afterthe scan paths 1-N are filled with stimulus and emptied of response,tester 1008 transitions to the capture state 1106 to load the nextresponse data, then returns to the operate state 1104 to input the nextstimulus data and empty the next response data. After all stimulus andresponse data patterns have been applied, by repeating transitionsbetween the operate and capture states, the test is complete and thetester returns to the idle state 1102.

The structure and operation of the parallel scan architecture of FIG. 10is very similar to the structure and operation of the single scanarchitecture of FIG. 1. Some of the most notable differences between thescan architectures of FIGS. 1 and 10 include. (1) In FIG. 10, multipleparallel scan paths 1-N are formed during the test configuration, asopposed to the single scan path 104 formed during the FIG. 1 testconfiguration. (2) In FIG. 10, tester 1008 outputs multiple parallelstimulus outputs 1018-1024 to scan paths 1-N, as opposed to tester 110outputting a single stimulus output 118 to scan path 104. (3) In FIG.10, tester 1008 inputs multiple parallel response outputs 1026-1032 fromscan paths 1-N, as opposed to tester 110 inputting a single responseoutput 120 from scan path 104.

The parallel scan architecture of FIG. 10 suffers from the same powerconsumption problem described in the scan architecture of FIG. 1, sinceduring scan operations, logic 1006 receives simultaneous ripplingstimulus inputs from scan paths 1-N. Thus, the parallel scanarchitecture of FIG. 10 can be improved to where it consumes less powerduring test by adapting it into a low power parallel scan architectureas described below.

Low Power Parallel Scan Architecture

FIG. 12 illustrates the FIG. 10 parallel scan architecture after it hasbeen adapted for low power operation. The adaptation process, aspreviously described in the low power adaptation of the FIG. 1 scanarchitecture, involves the following steps. Step one includesreconfiguring scan paths 1-N 1010-1016 of FIG. 10 into scan paths 1-N1202-1208 of FIG. 12, wherein each scan path 1-N 1202-1208 containsmultiple separate scan paths between their respective inputs 1018-1024and outputs 1026-1032. In this example, it is assumed that each scanpath 1-N 1202-1208 has been reconfigured into separate scan paths A, B,and C, as scan path 104 of FIG. 1 was reconfigured into scan path 502 ofFIG. 5. Step two includes inserting adaptor 1210 between tester 1008 andscan paths 1-N 1202-1208. In this example, it is assumed that adaptor1210 is very similar to adaptor 504 in the way it operates the separatescan paths A, B, and C in each of the scan paths 1-N 1202-1208, so onlythe brief operation description of adaptor 1210 is given below.

As seen in the operation diagram of FIG. 13, adaptor 1210 responds totester 1008 entering the operate state 1104 to: (1) simultaneouslyoperate the scan paths A of scan paths 1202-1208, via control bus 1212,to input stimulus from tester 1008 and output response to tester 1008,then (2) simultaneously operate the scan paths B of scan paths1202-1208, via control bus 1212, to input stimulus from tester 1008 andoutput response to tester 1008, then (3) simultaneously operate the scanpaths C of scan paths 1202-1208, via control bus 1212, to input stimulusfrom tester 1008 and output response to tester 1008. Adaptor 1210suspends scan operations to scan paths 1202-1208 when tester 1008 entersthe capture state 1106, and resumes the above described scan operationsequence to the scan paths A, B, and C of scan paths 1202-1208 whentester 1008 re-enters the operate state 1104. After the test completes,tester 1008 enters the idle state 1102 and the adaptor 1210 is disabled.From this description, the operation of adaptor 1210 is seen to mirrorthe operation of adaptor 504 with the exception that adaptor 1210controls multiple scan paths A, multiple scan paths B, and multiple scanpaths C during its control state diagram sequence 1302. In contrast,adaptor 504 controlled only one scan path A, one scan path B, and onescan path C during its control state diagram sequence 602.

Direct Synthesis of Low Power Scan Architectures

While the process of adapting pre-existing scan architectures for lowpower operation has been described, it is anticipated that, once the lowpower benefit of the present invention is understood, test synthesistools will be improved to provide direct synthesis of low power scanarchitectures. Direct synthesis of low power scan architectures willeliminate the need to perform the adaptation steps previously described,since the steps will be incorporated into the synthesis process. Adirect synthesis of a single scan path low power scan architecture wouldresult in the direct instantiation of a low power scan architecturesimilar to the one described and shown in regard to FIG. 5. A directsynthesis of a parallel scan path low power scan architecture wouldresult in the direct instantiation of a low power scan architecturesimilar to one described and shown in regard to FIG. 12.

Adapting Scan Controller Architectures for Low Power Operation

FIG. 14 illustrates a circuit 1400 configured into a conventional scancontroller based scan architecture. The scan architecture consists oflogic 1410, scan paths 1412-1418, and scan controller 1402. The scanpaths are coupled to logic 1410 via stimulus and response paths 1424, toscan controller 1402 via path 1404, and to tester 1408 via scan inputs1420 and scan outputs 1422. The scan controller is coupled to tester1408 via path 1406. While the scan controller based architecture of FIG.14 uses parallel scan paths 1412-1418, a single scan path architecture,such as the one shown in FIG. 1, could be used as well. The scanarchitecture operates to test logic 1410 as previously described inregard to the scan architecture of FIG. 10, with the exception that thetester 1408 inputs control to scan controller 1402 instead of directlyto the scan paths. In response to tester control input, the scancontroller outputs scan control 1404 to scan paths 1412-1418 to executethe test. An example control diagram for the scan controller is shown inFIG. 15. While various different control diagrams for various differentscan controllers could be shown, the diagram of FIG. 15 reflects thebasic scan operations typically required by any scan controller circuit1402. Those operations being, an idle state 1502, an operate scan state1504, and a capture response state 1506. It is understood that variousother scan operation states could exist in the control diagram.

The scan controller of FIG. 14 could be anyone of many types of scancontroller circuits. Two examples of some of the types of scancontrollers that could be represented by scan controller 1402 are listedbelow.

In one realization, scan controller 1402 could represent the test accessport (TAP) controller circuit of IEEE standard 1149.1, A Standard TestAccess Port and Boundary Scan Architecture. A description of the IEEETAP being used to control scan access to parallel scan paths isdescribed in regard to FIG. 14 a of U.S. Pat. No. 5,526,365 by Whetseland is incorporated herein by reference. The TAP operation states differfrom the operation state diagram of FIG. 15, but in general it containsthe fundamental scan 1504 and capture 1506 states.

In another realization, scan controller 1402 could represent theboundary input/output serializer (BIOS) circuit, described in regard toFIG. 17 of the above mentioned U.S. Pat. No. 5,526,365, being used tocontrol scan access to parallel scan paths. The BIOS description in U.S.Pat. No. 5,526,365 is incorporated herein by reference. The BIOSoperation also differs from the operation state diagram of FIG. 15, butin general it contains the fundamental scan 1504 and capture 1506states.

In still another realization, scan controller 1402 could represent theaddressable test port (ATP) circuit, described in TI patent applicationTI-28058, being used to control scan access to parallel scan paths. TheTI-28058 patent application is incorporated herein by reference. As withthe TAP and BIOS, the ATP operation differs from the operation statediagram of FIG. 15, but in general it contains the fundamental scan 1504and capture 1506 states.

FIG. 16 illustrates the two modification steps to the scan controllerbased scan architecture of FIG. 15 to achieve the desired low power modeof operation. In the first modification step, as with the previouslydescribed modification of the FIG. 10 scan architecture into the FIG. 12low power scan architecture, each of the scan paths 1412-1413 of FIG. 14are converted into low power scan paths 1602-1608. Each of the low powerscan paths 1602-160.8 of FIG. 16 contain separate scan path segments A,B, and C arranged as shown and described previously in regard to FIG. 5.

In the second modification step, an adaptor 1610 is inserted between thescan controller 1402 and scan paths 1602-1608. Adaptor 1610 inputscontrol from scan controller 1402 via path 1404 and outputs control toscan paths 1602-1608 via path 1612. From the general control statediagram example shown in FIG. 17, adaptor 1610 responds to scancontroller 1402 output states (idle state 1502, operate scan paths 1-Nstate 1504, and capture response data state 1506) to output controlstate sequences 1702 (operate scan paths A 1704, operate scan paths B1706, and operate scan paths C 1708) to scan paths 1602-1608. As withprevious adaptor descriptions, the control output from adaptor 1610 toscan paths 1602-1608 operates the scan paths 1602-1608 such that onlyone of the scan path segment groups (i.e. segment group A, B, or C) ofscan paths 1602-1608 are enabled to shift data at a time.

Since the adaptor's 1610 control input 1404 may come from any type ofscan controller, such as the TAP, BIOS, or ATP mentioned above, theadaptor 1610 design will need to be customized to interface with thespecific scan controller 1402 being used. In general, an adaptor 1610can interface to any given scan controller by simply sensing when thescan controller starts a scan operation and sensing when the scancontroller stops a scan operation. For example, when a scan controllerstarts a scan operation the adaptor starts executing its operate scanpaths A, B, and C state sequence, and when the scan controller stops ascan operation the adaptor stops executing its operate scan paths A, B,and C state sequence.

While FIG. 16 illustrates the adaptor 1610 as being a circuit separatefrom scan controller circuit 1402, the two circuits can be designed asone circuit. For example, if it is desired to provide the low power scanmode of the present invention in a scan controller based architecture,the scan controller and the adaptor circuit functions may be integratedinto a single circuit realization. The previously mentioned IEEE 1149.1TAP scan controller may indeed be designed to include the adaptor's 1610scan path segment A, B, and C sequencing functionality. Likewise, thepreviously mentioned BIOS or ATP scan controllers may indeed be designedto include the adaptor's 1610 scan path segment A, B, and C sequencingfunctionality.

Daisychained Low Power Scan Paths

FIG. 18 illustrates an IC 1800 having three intellectual property corecircuits (core1, core2, core3) 1802-1806 configured into a daisychainedarrangement for simultaneous parallel scan testing. Cores 1-3 could eachbe a DSP, CPU, or other circuit type. In the scan test configuration,each core includes a logic circuit to be tested, and N scan paths forcommunicating stimulus and response test patterns to the logic circuit.The scan paths 1-N of cores 1-3 are assumed to have the same length. Thescan inputs of the core 1 scan paths are connected to a tester, such astester 1008 of FIG. 10, via scan input paths 1808. The scan outputs ofthe scan paths of core 1 are connected to the scan inputs of the core 2scan paths via connections 1814. The scan outputs of the scan paths ofcore 2 are connected to the scan inputs of the core 3 scan paths viaconnections 1816. The scan outputs of the core 3 scan paths areconnected to a tester, such as tester 1008 of FIG. 10, via scan outputpaths 1812. The scan paths of cores 1-3 are connected to a control bus1810 to synchronize their daisychained scan test operation. Control bus1810 could be the control bus 1034 of tester 1008 of FIG. 10, or itcould be the control bus 1404 of scan controller 1402 of FIG. 14.

During test operation, the scan paths of cores 1-3 are controlled torepeat the steps of; (1) performing a capture operation to load responsedata from their respective logic circuits, and (2) performing a shiftoperation to unload response data to the tester via path 1812 and loadthe next stimulus data from the tester via path 1808. The duration ofthe shift operation is such that all the daisychained scan paths areemptied of their captured response data and filled with their nextstimulus data. From inspection of FIG. 18 it is seen that during theshift operation, the logic circuits of cores 1-3 receive ripplingstimulus inputs which consumes power in the logic circuits.

FIG. 19 illustrates an IC 1900 having three intellectual property corecircuits (core1, core2, core3) 1902-1906 configured into a daisychainedarrangement for simultaneous parallel scan testing. This example isprovided to demonstrate that cores 1-3 could each have a differentnumber of scan paths when configured into the test mode. For example,core 1 has two scan paths, core 2 has three scan paths, and core 3 has Nscan paths. Scan paths 1 and 2 of cores 1-3 are serially connectedbetween the tester scan outputs 1914 and 1916 of connection 1908 and thetester scan inputs 1922 and 1924 of connection 1912. Scan paths 3 ofcore 2 and core 3 are serially connected between the tester scan output1918 of connection 1908 and the tester scan input 1926 of connection1912. Scan paths 4-N of core 3 are serially connected between the testerscan outputs 1920 of connection 1908 and the tester scan inputs 1928 ofconnection 1912.

As in the FIG. 18 example, each core 1-3 of FIG. 19 includes a logiccircuit to be tested, and stimulus and response connections between thelogic circuits and the respective scan paths of each core. As in FIG.18, the scan paths of cores 1-3 are assumed to have the same length. Thescan paths of cores 1-3 are connected to a control bus 1910 tosynchronize their daisychained scan test operation. As mentioned inregard to FIG. 18, control bus 1910 could come from a tester, such astester 1008 or from a scan controller, such as scan controller 1402.

During test operation, the scan paths of cores 1-3 are controlled torepeat the steps of; (1) performing a capture operation to load responsedata from their respective logic circuits, and (2) performing a shiftoperation to unload response data to the tester via path 1912 and loadthe next stimulus data from the tester via path 1908. The duration ofthe shift operation is such that the longest daisychained scan patharrangement (i.e. the daisychain arrangement of scan paths 1 and 2 ofcores 1-3) is emptied of captured response data and filled with nextstimulus data. The scan patterns communicated to the daisychained scanpaths 3 between tester connections 1918 and 1926 will be padded with bitpositions to balance their bit length to the bit length of the scanpatterns communicated to the daisychained scan paths 1 and 2 betweentester connections 1914, 1916, 1922, and 1924. Likewise, the scanpatterns communicated 1 scan paths 4-N between tester connections 1920and 1928 will be padded with additional bit positions to balance theirlength to the sit length of the scan patterns communicated to thedaisychained scan paths 1 and 2 between connections 1914, 1916, 1922,and 1924. From inspection of FIG. 19 it is seen that during the shiftoperation, the logic circuits of cores-1-3 receive rippling stimulusinputs which consumes power in the logic circuits.

Adapting Daisychained Scan Paths for Low Power Operation

FIG. 20 illustrates IC 200, which is the IC 1800 after the scar paths1-N of cores 1-3 of IC 1800 have been converted into low power scanpaths 1-N. Besides the conversion of the conventional scan paths 1-Ninto low power scan paths 1-N, the IC 2000 is the same as that of IC1800. The cores 1-3 of FIG. 20 are the cores 1-3 of FIG. 18. The logiccircuits of the cores 1-3 of FIG. 20 are the logic circuits cores 1-3 ofFIG. 18. The scan input connections 2008 of FIG. 20 are the scan inputconnections 1808 from the tester of FIG. 18. The scan output connections2012 of FIG. 20 are the scan output connections 1812 to the tester ofFIG. 18. The scan path connections 2014 and 2016 of FIG. 20 are the scanpath connections 1814 and 1816 of FIG. 18. The low power scan paths ofFIG. 20 are assumed to each be partitioned into separate scan pathsegments A, B, and C, as previously described in regard to thepartitioning of scan path 104 of FIG. 1 into low power scan path 502 ofFIG. 5. The scan paths of cores 1-3 of FIG. 20 are connected to acontrol bus 2010 to synchronize their daisychained scan test operation.The control bus 2010 of FIG. 20 differs from the control bus of FIG. 18in that it comes from an adaptor, such as from adaptor 504 of FIG. 5,adaptor 1210 of FIG. 12, or adaptor 1610 of FIG. 16.

During test operation, the low power scan paths of cores 1-3 arecontrolled to repeat the steps of; (1) performing a capture operation toload response data from their respective logic circuits, and (2)performing an adaptor controlled shift operation to unload response datato the tester via path 2012 and load the next stimulus data from thetester via path 2008. The adaptor control sequences through the operatescan paths A, B, and C states as previously described in regard to statediagram 1302 of FIG. 13. The duration of the adaptor controlled shiftoperation is such that all the daisychained low power scan paths areemptied of their captured response data and filled with their nextstimulus data. From inspection of FIG. 20 it is seen that during theadaptor controlled shift operation, the logic circuits of cores-1-3receive rippling stimulus inputs only from the currently shifting scanpaths A, B, or C of each low power scan path. Thus the power consumed bythe core 1-3 logic circuits in FIG. 20 during shift operations isreduced from the power consumed by the core 1-3 logic circuits of FIG.18 during shift operations.

FIG. 21 illustrates IC 2100, which is the IC 1900 after the scan pathsof cores 1-3 of IC 1900 have been converted into low power scan paths1-N. Besides the conversion of the conventional scan paths into lowpower scan paths, the IC 2100 is the same as that of IC 1900, includingthe scan path connections to each other and to the tester as describedin regard FIGS. 18 and 20 above. The low power scan paths of FIG. 21 areassumed to each be partitioned into separate scan path segments A, B,and C, as previously described in regard to the partitioning of scanpath 104 of FIG. 1 into low power scan path 502 of FIG. 5. The scanpaths of cores 1-3 of FIG. 21 are connected to a control bus 2110 tosynchronize their daisychained scan test operation. The control bus 2110of FIG. 21 differs from the control bus of FIG. 19 in that it comes froman adaptor, such as from adaptor 504 of FIG. 5, adaptor 1213 of FIG. 12,or adaptor 1610 of FIG. 16.

During test operation, the low power scan paths of cores 1-3 arecontrolled to repeat the steps of; (1) performing a capture operation toload response data from their respective logic circuits, and (2)performing an adaptor controlled shift operation to unload response datato the tester via path 2112 and load the next stimulus data from thetester via path 2102. The adaptor control sequences through the the scanpaths A, B, and C states as previously described in regard to statediagram 1302 of FIG. 13. The duration of the adaptor controlled shiftoperation is such that the longest daisychained low power scan pathconnector i.e. the scan path 1 and 2 connections between cores 1-3 areemptied of their captured response data and filled with their nextstimulus data. From inspection of FIG. 21 it is seen that during theadaptor controlled shift operation, the logic circuits of cores 1-3receive rippling stimulus inputs only from the currently shifting scanpaths A, B, or C of each low power scan path. Thus the power consumed bythe core 1-3 logic circuits in FIG. 21 during shift operations isreduced from the power consumed by the core 1-3 logic circuits of FIG.19 during shift operations.

As previously described in regard to the test times of using scan paths104 and 502 to test logic 108 of FIGS. 1 and 5, the test times of usingthe scan paths of FIGS. 18 and 20 to test the logic circuits of cores1-3 are the same, as are the test times of using scan paths of FIGS. 19and 21 to test the logic circuits of cores 1-3. Also, as previouslydescribed in regard to FIG. 9, the tester scan input and scan outputpattern frames used for testing the FIG. 18 cores can be directly reusedto test the FIG. 20 cores. Likewise, the test patterns used for testingthe FIG. 19 cores can be reused to test the FIG. 21 cores.

As mentioned, the scan paths of the FIGS. 18 and 19 circuits wereassumed to be of equal length (M). Also, the corresponding low powerscan paths of FIGS. 20 and 21 were assumed to be modified from the Mlength scan paths FIGS. 18 and 19 such that the scan path segments A, B,and C of each low power scan path are of equal length (M/3). With theseassumptions made, a single adaptor circuit can be used to operate allthe low power scan paths of cores 1-3 of FIGS. 20 and 21 during scanoperations. The single adaptor circuit would sequence through threecycles of scan burst time intervals 802, 804, and 806 of FIG. 8 duringeach scan operation. For example, using the scan operation timingdiagram of FIG. 8 modified as described below, the scan operation of theFIGS. 20 and 21 scan paths can be understood. In FIG. 8, when SCANENAgoes low at time 812 to start the scan operation, a first cycle of scanburst intervals 802-806 occurs, followed by SCANENA remaining low whilea second cycle of scan burst intervals 802-806 occurs, followed bySCANENA remaining low while a third cycle of scan burst intervals802-806 occurs. Following the third cycle of can burst intervals 802-806SCANENA returns high to end the scan operation.

While the adaptor may be designed to operate the low power scan pathdifferent from the operation described above, the above describedoperation maintains the ability to reuse the test pattern frames of theoriginal scan paths of FIGS. 18 and 19. For example, each of theexisting scan test pattern frames (stimulus and response) for the FIGS.18 and 19 circuit can be viewed in the format of“[core1][core2][core3]”, where [core1] indicates the scan frame bitpositions targeted for the core 1 scan paths, [core2] indicates the scanframe bit positions targeted for the core 2 scan paths, and [core3]indicates the scan frame bit positions targeted for the core 3 scanpaths. It follows from the previous description given for FIG. 9 that;[core1] can be farther viewed in a format of [C.sub.1B.sub.1A.sub.1],[core2] can be further viewed in a format of [C.sub.2B.sub.2A.sub.2],and [core3] can be further viewed in a format of[C.sub.3B.sub.3A.sub.3], where the C.sub.1,2,3, B.sub.1,2,3, andA.sub.1,2,3 subset bit positions are targeted for each low power scanpath segment C, B, and A of cores 1-3 of FIGS. 22 and 21. The abovedescribed adaptor scan operation would operate to load and unload eachsubset C.sub.1,2,3, B.sub.1,2,3, A.sub.1,2,3 scan frame bit positionsinto the respective C, B, and A scan path segments of each low powerscan path of FIGS. 20 and 21. The advantage to this, as mentioned inregard to FIG. 9, is that test pattern frames originally provided forthe scan path arrangement of FIGS. 18 and 19 do not have to modified foruse with the low power scan path arrangement of FIGS. 20 and 21.

As mentioned in regard to the FIG. 19 daisychain arrangement, lengthcompensating pad bit positions are included in the test pattern framescommunicated to the daisychained scan paths 3 of cores 2 and 3, and inthe test pattern frames communicated to scan paths 4-N of core 3. Duringthe adaptor operated scan operation of the FIG. 21 circuit, these padbit positions are communicated during the above mentioned cycles of scanburst timing intervals 802-806, such that at the end of each adaptorcontrolled scan operation, all scan path segments A, B, C of all lowpower scan paths properly filled with stimulus and emptied of response.

In FIG. 20, if the A, B, C segments of the low power scan paths of core1 have the same length, the A, B, C segments of the low power scan pathsof core 2 have the same length, and the A, B, C segments of the lowpower scan paths of core 3 have the same length, but the lengths of theA, B, C segments of the low power scan paths of cores 1-3 are not thesame, separate adaptor interfaces will be needed to control the lowpower scan paths of each core 1-3. A first adaptor interface will beconnected to the control input 2018 of core 1 to provide control of thecore 1 A, B, C segments, a second adaptor interface will be connected tothe control input 2020 of core 2 to provide control of the core 2 A, B,C segments, and a third adaptor interface will be connected to thecontrol input 2022 of core 3 to provide control of the core 3 A, B, Csegments. The use of separate adaptor interfaces allows each of the core1-3 low power scan paths to be operated according to the scan bursttiming intervals (802-806) required to communicate to each of thedifferent length low power scan path A, B, C segments of cores 1-3. Forexample, if the A, B, C segment lengths of cores 1, 2, and 3 were 100,300, and 900 respectively, the scan burst intervals (802-806) of core 1would be set at 100 each, the scan burst timing intervals (802-806) ofcore 2 would be set at 300 each, and the burst timing intervals(802-806) of core 3 would be set at 900 each. A single adaptor circuitmay be equipped with multiple separate interfaces for connection tocontrol inputs 2018-2022, or separate adaptor circuits may be interfacedto control inputs 2018-2022. In a daisychained arrangement, as in FIG.20, each core may have different A, B, C scan segment lengths. Howeverfor proper daisychain operation, each of the different core A, B, C scanlengths should be set to positive integer multiples of one another suchthat the adpator interfaces to each core can operate together duringeach scan operation cycle (i.e. from SCANENA going low at 812 to SCANENAgoing high at 814 in FIG. 8) to modulate test patterns through alldaisychained cores without loosing any of the stimulus and responsepattern bits. For example the 100, 300, and 900 A, B, C core scansegment lengths mentioned above have been set to where, during each scanoperation cycle, the adaptor of core 1 modulates test patterns throughcore 1 using multiple cycles of 100 bit scan burst intervals (802-806),the adaptor of core 2 modulates test patterns through core 2 usingmultiple cycles of 300 bit scan burst intervals (802-800), and theadaptor of core 3 modulates test patterns through core 3 using multiplecycles of 900 bit scan burst intervals (802-806). Since 900 is amultiple of 300 and 300 is a multiple of 100, all A, B, C scan paths ofcores 1-3 will be properly filled and emptied during each scan operationcycle.

Scalable Scan Architecture Power Consumption

As can be anticipated from the description given for the presentinvention, the power consumption of logic circuitry being tested by thelow power scan architecture decreases as the number separate scan pathswithin the low power scan paths increases. For example, configuring agiven conventional scan path into a low power scan path comprising twoseparate scan paths may reduce power consumption by up to 50%, since,during operation, each of the two separate scan paths separately chargeand discharge one half, potentially, of the logic circuitry capacitancecharged and discharged by the convention scan path. Further, configuringthe same conventional scan path into a low power scan path comprisingthree separate scan paths may reduce power consumption by up to 66%,since, during operation, each of the three separate scan pathsseparately charge and discharge one third, potentially, of the logiccapacitance charged and discharged by the convention scan path. Stillfurther, configuring the same conventional scan path into a low powerscan path comprising four separate scan paths may reduce powerconsumption by up to 75%, since, during operation, each of the fourseparate scan paths separately charge and discharge one fourth,potentially, the logic capacitance charged and discharged by theconvention scan path. From this it is seen that the present inventionallows a synthesis tool to be provided with the capability of scalingthe power consumption of a given synthesized scan architecture to meet adesired low power mode of test operation of a circuit.

Scalable Scan Architecture Noise Reduction

As can be anticipated from the description given for the presentinvention, the noise generated by logic circuitry being tested by thelow power scan architecture decreases as the number separate scan pathswithin the low power scan paths increases. For example, configuring agiven conventional scan path into a low power scan path comprising twoseparate scan paths may reduce noise generation by up to 50%, since,during operation, each of the two separate scan paths separatelyactivate only one half, potentially, of the logic circuitry activated bythe conventional scan path. Further, configuring the same conventionalscan path into a low power scan path comprising three separate scanpaths may reduce noise generation by up to 66%, since, during operation,each of the three separate scan paths separately activate only onethird, potentially, of the logic circuitry activated by the conventionscan path. Still further, configuring the same conventional scan pathinto a low power scan path comprising four separate scan paths mayreduce noise generation by up to 75%, since, during operation, each ofthe four separate scan paths separately activate one fourth,potentially, of the logic circuitry activated by the convention scanpath. From this it is seen that the present invention allows a synthesistool to be provided with the capability of scaling the noise generationof a given synthesized scan architecture to meet a desired low noisemode of test operation of a circuit.

Circuit 220 of FIG. 22 illustrates an alternative method of controllinglow power scan segments A 506, B 508, and C 510 during test. Circuit2200 is similar to circuit 500 of FIG. 5 with the following exceptions;(1) adaptor 504 has been replaced with a decode logic circuit 2201, (2)additional externally accessible control inputs C1 2202 and C0 2203 havebeen provided and connected to the decode logic 2201, and (3) the tester2204 has been equipped with outputs for controlling the C1 and C0inputs.

FIG. 23 illustrates the low power scan operation mode 2300 of circuit2200 when being controlled by decode logic 2201 via the scan interface114 and C1 and C0 inputs from tester 2204. At the beginning of the lowpower scan test, tester 2204 inputs scan control (SCANENA and SCANCK)and a 0:0 code on C1 and C0 to cause the circuit to transition from theidle state 2301 (functional mode) to the operate scan path A state 2302.In operation state 2302, scan path A 506 and its output buffer 512 areenabled by bus 518 from decode logic 2201 to shift stimulus data in andresponse data out via paths 118 and 120, respectively. When scan path Ahas been filled with stimulus and emptied of response, the tester inputsa 0:1 code on C1 and C0, respectively, which causes entry into operationstate 2303. In operation state 2303, scan path B 508 and its outputbuffer 514 are enabled by bus 520 from decode logic 2201 to shiftstimulus data in and response data out via paths 118 and 120,respectively. When scan path B has been filled with stimulus and emptiedof response, the tester inputs a 1:0 code on C1 and C0, respectively,which causes entry into operation state 2304. In operation state 2304,scan path C 510 and its output buffer 516 are enabled by bus 522 fromdecode logic 2201 to shift stimulus data in and response data out viapaths 113 and 120, respectively. When scan path C has been filled withstimulus and emptied of response, the tester inputs a 1:1 code on C1 andC0, respectively, which causes entry into operation state 2305. Inresponse to the 1:1 code, decode logic 2201 outputs control on bases518, 52C, and 522 to disable all output buffers 512-516 and cause allscan paths A, B, C 506-510 to capture response data from logic 108. Theabove described sequence of the tester is repeated for all required scanpattern cycles by the tester appropriately re-entering the C1 and C0code sequences of 0:0, 0:1, 1:0, and 1:1. At the end of test, the testreturns circuit 2200 to its functional mode by re-entering the idlestate.

Example Decode Logic Circuit

FIG. 24 illustrates an example decode logic circuit 2201 implementation.Decode logic 2201 inputs the SCANCK 212 and SCANENA 210 signals fromtester 2204, via bus 114. Decode logic 2201 outputs SCANCK-A signal 712,SCANCK-B signal 714, SCANCK-C signal 716, ENABUF-A signal 718, ENABUF-Bsignal 720, ENABUF-C signal 722, and the SCANENA signal 210. The SCANENAsignal 210 is connected to all scan cell 200 multiplexers 202 as shownin FIG. 2. The SCANCK-A signal 712 is connected, in substitution ofSCANCK signal 212, to all scan cell 200 D-FF 204 clock inputs of scanpath A. The SCANCK-B signal 714 is connected, in substitution of SCANCKsignal 212, to all scan cell 200 D-FF 204 clock inputs of scan path B.The SCANCK-C signal 716 is connected, in substitution of SCANCK signal212, to all scan cell 200 D-FF 204 clock inputs of scan path C. TheENABUF-A signal 718 is connected to the enable input of buffer 512. TheENABUF-B signal 720 is connected to the enable input of buffer 514. TheENABUF-C signal 722 is connected to the enable input of buffer 516.While 3-state buffers 512-516 are shown providing selectableconnectivity between the outputs of scan paths A, B, and C and bus 120,other switching circuits, such as multiplexers, could be used as well.

Decode logic 2201 includes combinational decode circuit 2401 and gates706-710. Decode logic 2201 inputs the C1 and C0 signals and outputs thescan clock enable signals 724-728 and buffer enable signals 718-722.During functional mode of circuit 2200, SCANENA is high as indicated attime 2501 in the timing diagram of FIG. 25. Also during functional modeof circuit 2200 at time 2501, the C1 and C0 signals are both high. WhileC1 and C0 are high, decode circuit 2401 outputs control signals 724-728that enable SCANCK to pass through gates 706-710 to functionally clockall D-FFs 204 of scan paths A, B, and C, via SCANCK-A, SCANCK-B, andSCANCK-C. In this example, the SCANCK is assumed to be the functionalclock during the functional mode of circuit 2200, and the test clockduring test mode of circuit 2200. Also while C1 and C0 are high, decodecircuit 2401 outputs control signals 718-722 to disable buffers 512-516.The scan operation mode is entered by tester 2204 inputting a low onSCANENA and a 0:0 code (i.e. lows) on C1 and C0, respectively, asindicated at time 2502 in FIG. 25.

Decode circuit 2401 responds to the 0:0 code on C1 and C0 to set ENACK-A724 and ENABUF-A to a state that enables SCANCK-A 712 and buffer 512.While C1:C0=0:0, a burst 2503 of SCANCK-A's is output from decode logic2201 to shift data through scan path A from path 118 to 120. When scanpath A has been shifted, tester 2204 inputs a 0:1 code on C1 and C0respectively. Decode circuit 2401 responds to the 0:1 code on C1 and C0to set ENACK-B 726 and ENABUF-B to a state that enables SCANCK-B 714 andbuffer 514. While C1:C0=0:1, a burst 2504 of SCANCK-B's is output fromdecode logic 2201 to shift data through scan path B from path 118 to120. When scan path B has been shifted, tester 2204 inputs a 1:0 code onC1 and C0 respectively. Decode circuit 2401 responds to the 1:0 code onC1 and C0 to set ENACK-C 728 and ENABUF-C to a state that enablesSCANCK-C 716 and buffer 516. While C1:C0=1:0, a burst 2505 of SCANCK-C'sis output from decode logic 2201 to shift data through scan path C frompath 113 to 120.

When scan path C has been shifted, tester 2204 inputs a high on SCANENAand a 1:1 code on C1 and C0. Decode circuit 2401 responds to the 1:1code on C1 and C0 to set ENACK-A, ENACK-B, and ENACK-C 724-728 to statesthat enable SCANCK-A, SCANCK-B, and SCANCK-C 712-716 to operatesimultaneously, and to set ENABUF-A, ENABUF-B, and ENABUF-C to statesthat disable buffers 512-516. While SCANENA is high and C1:C0=1:1 attime 2506, scan paths A, B, and C operate to capture response data fromlogic 108 during each SCANCK-A, B, and C clock. While the timing diagramof FIG. 25, illustrates two response capture operations occurring duringtime 2506, any number of response capture operations could be performed.At the end of the response capture operation, the tester 2204 repeatsthe above described input sequence on SCANENA and C1 and C0 to apply allremaining test patterns to logic 108.

Contrasting the decode logic 2201 controlled timing of FIG. 25 with theadaptor 504 controlled timing of FIG. 8, it is seen that the low powerscan test operation is similar in both. The primary difference is thatthe decode logic 2201 approach of FIGS. 22-25 require additional inputs(i.e. the C1 and C0 inputs) from tester 2204 to control when scan pathsA, B, and C are operated, whereas the adaptor 504 approach of FIGS. 5-8does not require the additional inputs from tester 110.

The decode logic 2201 of FIG. 24 can be easily described to an automatictest pattern generation tool, enabling the tool to easily generate thescan test patterns to be used by tester 2204. Since tester 2204 candirectly control the C1 and C0 inputs to select which scan path A, B, orC is selected for scanning, debug of a failing device, i.e. a failing ICor sub-circuit within an IC, is facilitated. For example, if a devicetest fails and it is desired to determine why the failure occurred, itis possible for the tester to directly and individually select one ofthe scan paths, say scan path A 512, so that a more exhaustive scan testcan be applied to the portion of the logic circuitry 108 associated withthe individually selected scan path A. Likewise, scan paths B and C canbe directly and individually selected to execute more exhaustive scantests on the portions of the logic circuitry 108 associated with thescan paths B and C. Furthermore, and by manipulation of the C1 and C0inputs, it is possible for tester 2204 to efficiently execute the stepsof; (1) individually selecting and scanning a stimulus pattern into scanpaths A, B, and C for input to logic 108, (2) performing one or morecapture operations as shown at tire 2506 of FIG. 25 to load scan pathsA, B, and C with response from logic 108, and (3) individually selectingand scanning the captured response patterns from scan paths A, B, and C.

Since the tester 2204 controls the C1 and C0 inputs, the scan paths A,B, C may be set to any desired bit length. For example, in FIG. 25,shift event 2503 may be set to 100 bits in length, shift event 2504 maybe set to 200 bits in length, and shift event 2505 may be set to 300bits in length. The different shift event bit lengths are easilycontrolled by the C1 and C0 inputs from the tester. However, while C1and C0 provide flexible control over shift event lengths, optimum scanpower reduction is achieved only when the shift event lengths are madeequal or near equal, as previously mentioned in regard to FIGS. 1 and 5.Making the shift lengths equal or near equal decreases the number of cancells 220 that are simultaneously accessed during each shift event,which reduces the number of rippling stimulus inputs to logic 108. Thusfor reducing power during scan test and irregardless of whether scancontrol comes from adaptor 504 of FIG. 5 or the decode logic 2201 ofFIG. 22, it is desirable to partition a conventional scan path up intolow power scan path segments that have equal or near equal bit lengths.

In FIG. 24, a SCANENA signal 2402 is shown being output from decodecircuit 2401 and connected, via dotted line, to the SCANENA signal 210.This is shown to simply indicate that, if desired, the SCANENA signal210 can be produced by decode of the C1 and C0 inputs. If decode circuit2401 provides an internally produced SCANENA signal 2402 and connectsthe signal to SCANENA 210, the tester does not need to provide anexternally produced SCANENA control input to the device via bus 114 ofFIG. 22. Using the timing diagram of FIG. 25, a 1:1 code on C1 and C0would set the SCANENA 2402 signal high at times 2501 and 2506, while allother codes (i.e. 0:0, 0:1, 1:0) would set the SCANENA 2402 signal lowat times 2503, 2504, and 2505 respectively. The low power scan operationdepicted in FIG. 25 using an internally produced SCANENA 2402 signal tocontrol the SCANENA 210 input to multiplexers 202 of FIG. 2 is identicalto using an externally provided SCANENA signal on bus 114 from a tester.The benefit of using an internally produced SCANENA signal 2401 is thatit allows for reducing the control input bus 114 from tester 2204 by oneinput.

In FIG. 25, it is seen that two external control inputs (C0-C1) can bedecoded into three individual scan path shift events 2503-2505 and onecapture event 2506. Thus two external control inputs provide control forup to three individual low power scan paths A, B, C. If more than threelow power scan paths are used, the number of control inputs would needto be increased to decode the additional shift events and capture event.For example, three control inputs (C0-C2) would be needed for a lowpower scan path arrangement that included seven separate shift eventsand one capture event.

FIGS. 26 and 27 are provided to illustrate that a single decode logic2201 and set of C1 and C0 inputs can be used to access a parallelarrangement of low power scan paths 2601-2604, similar to the way asingle adaptor 504 was illustrated doing so in FIGS. 12 and 13. Itshould be clear that decode Logic 2201 in combination with the C1 and C0control inputs could be substituted for adaptor 504 in all low powerscan examples described herein.

It should be clear that it is possible to eliminate decode logic 2201altogether if enough external control inputs are provided to allowtester 2204 to directly control the enabling of scan clocks 712-716 toscan paths A, B, and C via gates 706-710 and the switching of the scanpath A, B, and C serial outputs onto bus 120 via buffer 512-516. Forexample, in FIG. 24 the tester could provide separate control inputs forthe scan clock gating signals 724-728 and the scan path serial outputswitching signals 718-722. Furthermore, it is possible to provideexternal control inputs that would allow direct and separate control ofscan clocks 712-716, if desired. This expansion of control inputs wouldprovide greater flexibility in the operation and control of low powerscan path arrangements.

Although the present invention has been described in accordance to theembodiments shown in the figures, one of ordinary skill in the art willrecognize there could be variations to these embodiments and thosevariations should be within the spirit and scope of the presentinvention. Accordingly, modifications may be made by one ordinarilyskilled in the art without departing from the spirit and scope of theappended claims.

What is claimed is:
 1. An integrated circuit comprising: A. logiccircuitry having stimulus inputs and response outputs; B. a scan inlead, a scan out lead, a scan clock lead, and a scan enable lead; C. ascan path of serially connected scan cells, the scan path having aninput connected to the scan in lead, an output coupled to the scan outlead, each scan cell including: i. a multiplexer having one inputconnected to a response output, another input coupled to the scan inlead, a control input connected to the scan enable lead, and an output;and ii. a flip-flop circuit having an input connected to the output ofthe multiplexer, an input selectively coupled to the scan clock lead,and an output connected to a stimulus input and selectively coupled tothe scan out lead; D. the scan path being divided into segments ofserially connected scan cells, each segment including: i. a scan inputconnected to the scan in lead; ii. a separate scan clock input; iii. atristate output buffer selectively coupling a scan output with the scanout lead, the tristate buffer having a separate enable input; and E.decode logic circuitry having a scan clock input connected to the scanclock lead, a scan enable input connected to the scan enable lead,binary control inputs, and, for each segment, a separate scan clockoutput connected to the separate scan clock input, and a separate enablebuffer output connected to the separate enable buffer input.
 2. Theintegrated circuit of claim 1 in which the scan path has three segmentsand there are two binary control inputs for selecting the separate scanclock output and the separate enable buffer output for each segment. 3.The integrated circuit of claim 1 in which the scan path segments haveequal numbers of scan cells.
 4. The integrated circuit of claim 1 inwhich the scan path segments have unequal numbers of scan cells.